The present invention relates to a method for correcting the sensitivity of an electromagnetic ultrasonic probe and, more particularly, to a method for correcting the flaw detection sensitivity so that it is maintained at a fixed value when the distance between the electromagnetic ultrasonic probe and the material to be inspected varies.
In nondestructive testing, recently, ultrasonic flaw detection has been widely used because of its simple handling and high reliability. This flaw detecting method was usually applied to inspection of cold material. Recently, however, it has been found that this method is applicable to surface and internal flaw detection and measurement of the thickness of hot steel material during hot working operation. Since then, the ultrasonic flaw detecting method has also had practical application in use for inspecting hot steel for the purpose of the improvement of quality control and the economy of energy useage.
The following two methods are possible in ultrasonic flaw detection:
(1) ultrasonic waves are generated in a hot steel material by heat shock by heat irradiation of the material using a laser beam and received by an electromagnetic ultrasonic probe; and PA1 (2) ultrasonic waves are generated in a hot steel material by an electromagnetic ultrasonic probe and received by the same probe.
In the flaw detection using the electromagnetic ultrasonic probe, the material being inspected is not in direct contact with the probe but is spaced therefrom by a distance (hereinafter called lift-off) of 0.5-3 mm. Eddy currents are generated on the surface of the material being tested, by passing a high frequency pulse current to a coil of the probe. A vibrating force is generated on the surface of the material being inspected by Lorentz's force due to the static magnetic field on the top surface of the material and by the interaction of a magnet of the probe and the eddy current, thereby generating ultrasonic waves having a high frequency. The ultrasonic waves are propagated through the material being inspected, reflected by any source of reflection, and received as electric signals by the electromagnetic ultrasonic probe for reception.
Since the electromagnetic ultrasonic probe can produce sound waves which are different in their mode of vibration by changing the direction of the static magnetic field, it is possible to select a suitable type of vibration for the shape and direction of the defect.
In the ultrasonic flaw detection in which the electromagnetic ultrasonic probe is used for the detection of defects, it is essential that the flaw detection is performed with the initially determined reference sensitivity to the completion of the inspection. For this purpose, the lift-off must be maintained constant throughout the inspection. However, one of the problems encountered in the conventional ultrasonic flaw detection method was that it was impossible to maintain the lift-off constant and, accordingly, it was impossible to detect flaws with the same sensitivity because of the rough surface of the material being inspected, allowable error of the transfer line, and the scales attached thereto.
Accordingly, an object of the present invention is to provide a method for, even when the lift-off varies, correcting the flaw detection sensitivity to always maintain the initial reference sensitivity so as to always perform flaw detection at a constant sensitivity.